I've been watching airliners through my telescopes for years, before I had a flight tracker at my disposal to know exactly what I was looking at. It's fun. I wish I would have had FlightAware back when I first started.

The short answer to your question is "maybe". The video posted about gives you a good idea of what a telescope will show you under good conditions. It's tough to track an airline smoothly, but you can see things. With clear skies, it's pretty easy to decipher the operator and airplane type if you recognize it. The closer it is, the more detail you can spot. If a plane is right overhead, even at high cruise you might be able to read the registration if the carrier has it under the wing. Things like that are fun challenges.

Why not? Depending on the aircraft and airline (logo placement and size varies) and the visibility, I don't see why not. I have a Panasonic FZ70 that has a lens that equates to 20-1200mm, and despite the poor resolution due to the digital zoom, I can often read registrations of aircraft overhead. The logos are clearer for aircraft that are between 200 and 280 than they are for aircraft between 300 and 410 though, of course.